NI-5122 measure trigger frequency of TTL trigger signal

I have two signal channels and a TTL trigger signal on my NI-5122. The frequency of the tigger signal is between 5 and 30 Hz. Before I want to sample the two signal channels I need to know the actual trigger frequency to calculate and set sampling rate and number of sampling points. I use LabView 7.1
What's the best way to measure the frequency (5-30 Hz) of the TTL signal connected to the 5122 trigger input?
many thanks for your help.
Markus

Jason V,
thanks for your help. So there is no programable counter on the 5122...
My freuqency is constant and I need an accuracy of <1%.
What I did meanwhile:
I make a loop of e.g. 10 triggerevents.
I record the times when the trigger events occur.
After the loop I fit a line to the time vs. iteration number.
This gives a pretty good value for the period, hence I get the inverse frequency.
I need 10-30 trigger events to be accurate.
For 10 Hz this makes 1-3 seconds init at the start of each measurement.
I can reach a relative error as low as 1e-4. That's o.k.
I did some more tweaking to allow for missed trigger events.
The whole VI works fine to retrieve frequencies up to 100 Hz.
For higher frequencies I am limited due to the millisecond resolution of LabViews 'Tick' vi.
====
For higher frequencies I would program a counter:
Start a measurement for 1000 scans and measure the time for that.
However, this is not accurate enough for low frequencies and short recording times (10-30 trigger events).
====
I also thought about Fourier transforming the signal itself and locating the peak frequency.
Same as above: This is only accurate when I record many periods.
====
The fastest and most accurate solution would be to sample the trigger signal itself.
Then I would need only one period and could detect the trigger slopes with microsecond resolution.
However, I need both input channels for recording simultaneous signals.
Conclusion:
(1) The NI-5122 does not has a programable counter on board
(2) For my application I found a solution tht works.
MarkusMessage Edited by Nepu on 03-09-2005 02:04 AM
Message Edited by Nepu on 03-09-2005 02:07 AM

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Windows 2000,Pentium III,~800-900MHz,32-bit master PCI bus,256 MB RAM
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PCI-GPIB card
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Please see attached program flow diagram.
Qustions:
1. When I want to acquire 100 waveforms using 100 subsequent trigger pulses, the attached VI (Multiple_record VI) gives me the following error message:
ERROR
"Possible reason(s):
Driver Status: (Hex 0xBFFA4009)
A previous acquisition is still in progress. If you are attempting to change an attribute, note you can only change fetch attributes while an acquisition is still in progress.
Status Code: -1074118647
What am I doing wrong? Is there a better and more efficient way to do this?
2. How can I transfer the 100 waveforms from the digitizer, one-by-one as 1D-arrays, through my fitting routine? A waveform will have ~2000-3000 data pts. (DBL type). Remember: I DO NOT HAVE TO SAVE THE WAVEFORMS!
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a) Start transfer of already acquired waveforms (1D-arrays), one-by-one through the fitting routine, during the waveform acquisition process is still in progress?
b) First, acquire the 100 waveforms (1D-arrays), store them in the onboard memory (digitizer) and then transfer them one-by-one through the fitting routine?
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Attachments:
program flow diagram.vi ‏11 KB

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This account is no longer active. Contact ShadesOfGray for current posts and information.

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